This invention is concerned with improvements in and relating to instruments and methods of measuring, particularly, but not exclusively with regard to neutron radiation.
Accurate measurement of neutron dosimetry is important in ensuring that environments intended for operations involving personnel access are accurately surveyed and/or that personnel operating in environments where significant radiation exists receive a dose suitably below the limit permitted.
In most cases the dose is made up of neutrons of different energies forming a spectrum. The spread of this spectrum and the proportions of the dose in each part are significant because of the differing level of biological hazard they present.
In general in existing instruments the neutrons are detected and the results presented as a total dose equivalent. The relative hazard of different energies varies, however, and so a weighting is applied to the results or the instrument is designed to obtain an approximate weighting by physical means, in obtaining this equivalence. A typical indication of the weighting given to the different energy levels is indicated in FIG. 1. Prior art devices face a number of problems including the following.
Firstly many prior art instruments are limited in terms of the range of neutron energies they can monitor. Problems particularly occur at the high energy range. Such instruments are typified by a polyethylene sphere with a central detector. The volume of polyethylene necessary to attenuate high energy neutrons is prohibitive in terms of weight and size. Such devices, therefore, detect neutrons in part of the energy range with low efficiency.
Secondly the sensitivity of prior art instruments unfortunately varies across the energy range of interest. Thus an instrument may pick up a value of counts/nSv a factor of 6-10 different (peak to trough) at different neutron energies across the range. Such a sensitivity profile is illustrated in FIG. 2 for a prior art device. Thus if the device is calibrated at high energy levels the reading in the presence of a lower energy source can be quite erroneous. This can lead to access problems being identified for areas which are not actually subjected to high radiological dose or to unduly low values being determined. The use of supplemental spectroscopic measurements to establish the spectrum of neutrons detected and so apply an absolute correction factor is time consuming, expensive and impracticable.
These inaccuracies, and others, have been tolerated in the past as to an extent it was thought the worst deviations occurred in parts of the neutron energy range which were not of the greatest importance or where little dose was received. In recent times however it has been realised that the neutron risk factor is greater than previously accepted (a weighting of 20 for instance, rather than 10 relative to gamma photons is now applied at some energies) As a result increased sensitivity with less deviation throughout a wide energy range is sought.
Additionally as a third problem, the weighting factors applied to different parts of the spectra are periodically updated. The nature of many prior art instruments means that such changes would require wide scale reconfiguration in the device, even necessitating hardware replacement in some cases or acceptance of larger deviations. A more versatile instrument is thus also desirable.
The device should desirably be readily portable in terms of size and weight. Hand held applications are common and the operator is frequently called upon to hold the device at arms length to avoid interactions caused by the operator""s body.
According to a first aspect of the invention we provide an instrument for detecting radiation, the instrument comprising an inner neutron detector and an outer neutron detector, the inner and outer detectors being separated by an inner layer of neutron attenuating material, an outer layer of neutron moderating material being provided around the outer detector.
The instrument is preferably a survey instrument. The instrument is preferably used away from the operator. A separation of at least 50 cm, more preferably at least 1 m and ideally far greater may be employed.
Attenuation and/or moderation for the neutrons detecting is preferably only provided by the instrument, for instance when considered relative to ambient ions in the environment.
Preferably the inner layer of neutron attenuating material substantially or completely surrounds the inner detector. Preferably the inside of the inner layer conforms to the outside of the inner detector.
Preferably the outer layer of neutron moderating material substantially or completely surrounds the inner layer and/or outer neutron detector. Preferably the inside of the outer layer conforms to the outside of the inner layer and/or outer neutron detector.
The outer neutron detectors may be provided in or on a continuous or substantially continuous neutron detector carrier layer. The neutron detector carrier layer may be sandwiched between the inner and outer layers.
Preferably the inner layer is spherical. Preferably the inner surface of the inner layer is defined by a radius of between 1 and 8 cm, more preferably 2 to 5 cm. Preferably the outer surface of the inner layer is defined by a radius of between 3 and 13 cm, most preferably 4 to 8 cm.
Preferably the outer layer is spherical. Preferably the inner surface of the outer layer is defined by a radius of between 3 and 13.5 cm. The outer detector carrier layer maybe between 0.05 and 0.5 cm. Preferably the outer surface of the outer layer is defined by a radius of between 7 and 25 cm.
The inner detector may be cylindrical of cross sectional radius between 2 and 5 cm and of length between 5 and 15 cm.
The inner layer may be defined as a hollow cylinder with or without hollow hemispherical ends.
The inner surface of the cylindrical portion may be defined by a radius of between 1 and 5 cm. The outer surface of the cylindrical portion maybe defined by a radius of between 4 and 7.5 cm. The hemispherical end""s inner surface maybe defined by a radius of between 1 and 5 cm. The hemispherical end""s outer surface may be defined by a radius of between 4 and 7.5 cm.
The inner surface of the inner layer may be defined by a right cylinder of total length between 5 and 15 cm and/or of a radius between 1 and 5 cm. The outer surface of the inner layer may be defined by a right cylinder of between 10 and 25 cm in length and/or of radius between 4 and 7.5 cm.
The outer layer may be defined as a hollow cylinder with or without hollow hemispherical ends.
The inner surface of the cylindrical portion may be defined by a radius of between and 4 and 8 cm. The outer surface of the cylindrical portion may be defined by a radius of between 8 and 16 cm. The hemispherical end""s inner surface maybe defined by a radius of between 4 and 8 cm. The hemispherical end""s outer surface maybe defined by a radius of between 8 and 16 cm.
Alternatively the hemispherical end""s inner surface may be defined by a right cylinder of radius between 4 and 7.5 cm and/or with a length into the hemisphere of between 4 and 9 cm.
Preferably the inner layer comprises a plurality of layers. The layers may be of different materials and/or be of different properties. Preferably the inner layer comprises a first and second layer. Preferably the inner surface of the second layer conforms to the outer surface of the first layer. The first and second inner layers may be spherical or present as a hollow cylinder with or without hemispherical ends or as a right cylinder. The second layer may be thicker than the first and outside the first. The inner layer may be of composite form. The layers may both be of substantially constant thickness throughout. The thickness of the two layers may be the same or different to one another.
Preferably the outer layer is formed from a hydrogen containing material. Preferably the material is a plastic, such as polythene. The use of high density polythene is particularly preferred, for instance having a density of between 0.85 and 95 g/cm3.
The material maybe between 3 and 8 cm thick. The material may have a constant thickness, plus or minus 0.2 cm.
Preferably the outer layer is shaped to provide a substantially even thickness of material around the outer detectors. The outer layer may be spherical or cylindrical with hemispherical ends.
The outer layer may be provided with a carrying handle and/or externally mounted processing means. The processing means may alternatively be positioned away from the device. The processing means are preferably in communication with the device. Hard wiring, optical, radio or other means maybe used.
Preferably the thickness of material around the inner detector is substantially constant throughout, plus or minus 0.2 cm.
The inner layer, or one layer of it, may be formed from boron The boron may be natural or enriched in the boron 10 isotope, or in powder form or in a matrix. A plastics matrix maybe used to provide the boron.
The inner layer, or one layer of it, may be formed from a hydrogen containing material. The material may be a plastic, such as polythene.
Preferably the inner layer includes a boron containing layer and a plastics containing layer. It is particularly preferred that the plastics layer be provided innermost. Preferably the boron layer ranges between 1 and 5 cm in thickness. Preferably the plastics layer ranges between 0.5 and 3 cm in thickness.
The inner detector may be spherical or cylindrical. A sphere of between 1 and 8 cm, more preferably 1 and 5 cm, radius may be provided. The cylinder may be between 5 and 15 cm long. The cylinder maybe between 1 and 5 cm in radius.
The detector may be of the 3He type, with or without CH4, or of the BF3 type or comprise a scintillator with separate or incorporated 6Li converter and photo multiplier tube. A 3.5 atm 3He, 1 atm. or BF3 detector is a further option.
The signals from the inner and outer detectors are preferably conveyed separately to the processing means.
Preferably the instrument is capable of detecting neutrons in the energy range 1 eV to 20 MeV. Preferably the instrument is capable of detecting neutrons in the energy range thermal to 15 MeV. Preferably the outer detector is capable of detecting neutrons in the range thermal to 100 keV. Preferably the inner detector is capable of detecting neutrons in the energy range 100 keV to 15 MeV.
Preferably the outer detectors are solid state detectors. A 6Li converter in contact with a solid state electronic detector is particularly preferred.
Preferably a plurality of outer detectors are provided. Between 4 and 16 outer detectors may be used, with between 6 and 8 being particularly preferred. The detectors maybe provided in tetrahedral, orthogonal or octagonal distribution, most preferably offset from the vertical. Preferably the detectors are provided in an even distribution, without providing a detector under a handle or other potentially interfering part of the device.
The detectors may be sandwiched directly between the inner and outer layers or placed at a depth within the outer layer. Alternatively the detectors may be provided in a discrete layer of their own. The layer may be between 0.02 and 0.5 cm in thickness.
Preferably the instrument can detect neutrons in a substantially non-directionally sensitive manner.
Sensitivity in at least a 300xc2x0 arc and more preferably a 360xc2x0 arc, on an horizontal plane, about the vertical axis of the instrument, is preferably substantially equivalent throughout.
Sensitivity in at least a 70xc2x0 arc, more preferably an 80xc2x0 arc and ideally a 90xc2x0 arc extending from an horizontal plane for the instrument towards the vertical axis, upward or downward, is preferably substantially equivalent throughout. Preferably the equivalent sensitivity from the horizontal plane towards the vertical axis extends for at least a 300xc2x0 arc and more preferably a 360xc2x0 arc about the vertical axis of the instrument. The sensitivity in an arc downward from the horizontal plane may be the same, less or greater than for an arc upward from the horizontal plane.
Substantially equivalent sensitivity may be within + or xe2x88x9210% of the sensitivity of the average sensitivity of the instrument throughout that arc and/or 3-dimensional zone defined by the arc definitions. The sensitivity may be an overall average sensitivity for the energy spectrum measured and/or the sensitivity to one or more neutron energies, for instance, 10xe2x88x925 MeV, 10xe2x88x924 MeV, 10xe2x88x923 MeV, 10xe2x88x922 MeV, 10xe2x88x921 MeV, 1 MeV or 10 MeV.
The inner and outer detector signals are preferably processed separately. The processing means may provide a read out of dose equivalent rate and/or total dose equivalent and/or time of reading and/or spectral hardness. Preferably the readout is based on at least 100 counts.
According to a second aspect of the invention we provide a method of detecting radiation using a device according to the first aspect of the invention.
Various embodiments of the invention will now be described with reference to the drawings in which:
FIG. 1 shows fluence to dose equivalent conversion coefficients for different energy neutrons under old and updated coefficients;
FIG. 2 displays the relative neutron dose equivalent sensitivity for a prior art instrument;
FIG. 3 illustrates a first embodiment of an instrument according to the invention;
FIG. 4 illustrates a second embodiment of an instrument according to the invention;
FIG. 5 illustrates the relative dose equivalent sensitivity for an instrument according to the invention in comparison with prior art instruments;
FIG. 6 illustrates the detection rate against actual neutron fluence rate with energy for the inner and outer detectors of an instrument according to the invention;
FIG. 7 shows the dose equivalent response characteristics of inner and outer detectors against energy for the device of Example A; and
FIG. 8 shows the combined detector dose equivalent response characteristic against energy for the devices of both Example A and Example B.